An internal mode switch (IMS) is a component of a transmission used to relay the driver's selected range intent to a control system of a vehicle. The IMS is a sliding contact electrical switch assembly attached to a lower control valve body of the transmission. Conventional switches consist of two major components: a housing, which houses at least six tracks of conductive and non-conductive material that make up stationary contacts, and an insulator assembly, which makes up the moving contacts and is linked to an operator controlled shift lever. The six tracks include circuit A, circuit B, circuit C, circuit P, a park-neutral circuit, and a ground circuit.
The IMS is electrically connected to a control system of the vehicle via at least five wires (voltage is supplied to four and one is ground). Range detection is accomplished by securing the moving contacts of the IMS over the stationary contacts. For example, when a driver selects a PRNDL position, the shift lever causes IMS moving contacts to slide, which in turn grounds the four wires in a unique pattern for each PRNDL position. The bit pattern indicating the PRNDL position is comprised of A, B, C, and P circuits. The pattern is electronically communicated to the control system for interpretation.
The pattern provided to the control system is used for engine controls as well as determining transmission shift patterns. The input voltage level is high (ignition voltage) when the circuit is open and low when the circuit is closed to ground. It can be foreseen that a failure may occur in the IMS such that an inaccurate pattern is sent to the control system. A bit pattern strategy should be employed to detect all single point failure modes that may occur.